1. Technical Field
The present invention relates to a leveling system for the automatic sensing and adjusting of the inclination of equipment, such as assembly jig frames and fixtures used for the assembly and fabrication of sections of aircraft fuselage.
2. Background Art
In the aerospace industry, large airframe components are assembled by attaching individual piece parts to a structure called an assembly jig frame in a precise relationship to one another. The jig frame consists of adjustable lifting devices that may be connected by generally horizontal members, which support manufacturing planes. The manufacturing planes are generally positioned in a vertical relation to the horizon, but may be positioned in other suitable orientations for specific assembly operations. The piece parts or work pieces are held in place by tools that are, in turn, secured to the manufacturing planes. After the piece parts are fastened in the assembly jig frame, assembly operations are performed on the parts such as drilling and riveting or bonding.
The tools must be precisely located and attached to the assembly jig frame in order to assure the proper relationship between piece parts for the assembly operations. Precise location of the tools is also important because their relationship may have to be changed during assembly operations. For example, it is often necessary to remove one set of tools and install another set to complete an assembly operation, to assemble another section of the airframe component, or to perform a different operation on the same component.
Typically, precision optical measuring instruments identify the proper location for the tool details, and locate the tool details, on the assembly jig frame. Such precision instruments use their relationship with the horizon as the foundation for accurate measurements. Each such optical instrument used in the construction, attachment, and maintenance of the tool details located on the assembly jig frame is leveled, or made parallel to, the horizon before it is used. All location calculations are based on this datum plane. For this reason, the horizon is the most closely monitored and maintained datum plane in the manufacture and cycle calibration of the tool details located on, and attached to, the assembly jig frame.
Since these optical instruments use the horizon as the datum plane, the assembly jig frame must also use the same datum plane in order to establish and maintain a common relationship between the measuring instruments and the tools that are secured to the assembly jig frame. Therefore, it is critical that the assembly jig frame can be adjusted to attain, maintain and retain a predetermined alignment with the horizon.
Previous methods and devices used to align the assembly jig frame to the horizon are manually performed and adjusted. The assembly jig frames are positioned on cement floor and brought into a level position with the horizon through the use of adjustable lifting devices, such as jacks, attached to the bottom of the assembly jig frame. The jacks press against pads which sit on the cement floor. Each assembly jig frame may have as many as 30 jacks, and each jig frame may weigh up to 250 tons.
The position of the jig frame is adjusted by a team of mechanics. The team arbitrarily selects one jack with a close proximity to the floor, e.g., two inches, and identifies it as the "master jack", or reference jack. The purpose for this selection is to minimize the upward adjustments of all jacks by identifying the jacks to be adjusted as those that require the minimum travel to achieve horizontal, starting with the reference jack.
The process begins with one mechanic turning a jack by hand using a wrench while a second mechanic holds a measuring scale positioned on a horizontal portion of the frame, directly above the jack. A third mechanic uses an optical instrument, such as a level, to measure and record the distance from the level of the instrument to the selected part of the frame, for example a level button. The first and second mechanic then move to a second jack and adjust its height until the third mechanic signals that the same height is indicated. The order in which the jacks are selected for adjustment is determined empirically for a particular type of jig frame. This method is repeated sequentially on each jack until all the horizontal members supporting the assembly jig frame are all in the same datum plane, i.e., parallel to the horizon.
The next step in initially setting up the jig frame for assembly operations is to measure the manufacturing planes of the jig to determine if the planes are in desired orientations for assembly operations. Although the jig frame may be assembled with the manufacturing planes in the desired orientation to the horizontal members, this assembly operation does not result in alignments with the degree of precision adequate for assemble operations, e.g. thousands of an inch for manufacturing planes of 25 to 50 feet in length.
To correct for this misalignment, the jacks must be adjusted to bend the jig frame very slightly in a precise manner to bring the manufacturing planes into proper alignment. For example, if a manufacturing plane is required to be vertical to the horizon, an optical instrument, such as a jig transit, is used to site selected locations on the plane, i.e., tooling pads, and determine the distances that the pads must be moved to bring the manufacturing plane into a vertical orientation to the horizon. To achieve this orientation, mechanics then adjust jacks to raise or lower portions of the jig to bend the jig frame until sequential observation of the tooling pads by the jig transit indicates that the manufacturing plane is vertical to the horizon. The jig may then be ready to start assembly operations.
Subsequent adjustments of the jig may be needed depending on the type of assembly operations to be performed. For example, one of the manufacturing planes may need to be moved to a different orientation than its original orientation in order to accomplish a selected assembly operation and then returned to its original orientation. This method could be required in order to assemble a component into a airframe that has been stressed in order to induce a residual stress in the component after the stress has been removed from the airframe. To accomplish this with current techniques, repeated observation of tooling pads and adjustment of the jacks is required , i.e. a "trial and error" method.
After initial set up, the observation and adjustment operation must be repeated approximately every six months or whenever a physical trauma has occurred that may affect the level of the assembly jig frame. Such a trauma may result from natural causes as an earthquake, aftershocks or other geologic events including tidal ebb and flow, i.e., sublimation. Adjustment is also necessary because of the additional weight of aircraft components that are added to the jig frame during aircraft construction. Each such manual adjustment cycle of the jig frame results in costly disruption of the aircraft assemble operation.